I`d go with a Luxfer AL 40cft bottle with a DIN valve; this is the size you’ll use for more advanced training. As for bottle position I wear mine on the left side so it won`t interfere with then scooter and can light. Also wearing it on the left side facilitates valve manipulation and streamlining. I`d stay away from mounting it to your tanks behind you because you cannot see if there is a problem. I had one guy give me OOG signal on a deco hang because his back mounted pony had a leak and was empty. Here is asite that shows how to rig a deco/stage bottle. http://dir-diver.com/en/equipment/stagebottle_rigging.html Cheers, Vinny